Optimal Apex Radius would be the thinnest which sustain for intended usage. It would be inefficient to use a jack hammer bit keennes as a tomato slicer, flip around is unsustainable to use a gyuto sub micron edge for jack hammering. Both too thick & too thin lead to bad performance and often damage to tools.

We can calculate PSI (or Pascal in metric) for work and edge interaction. Just tune to apex radius to PSI range which this edge can support/operate successfully.

E.g. if an sharpened edge with 250nm apex and psi exceed this edge strength, it is more optimal to flatten the apex to wider width or put on a 90 degrees micro bevel (works almost as a wider apex). Do so until, apex can sustain interaction PSI. It makes sense not to put a shaving razor edge to a chopper for chopping hardwood.

Keep in mind, PSI refer to interaction surfaces, while impact is the load which isn’t addressed here. Think PSI akin to Voltage, where impact load is equivalent to Current.